The steering system of an engineering vehicle such as a wheel loader includes: a hydraulic pump; a steering cylinder driven by pressurized oil discharged from the hydraulic pump; and a steering valve for controlling the direction and flow rate of the pressurized oil supplied from the hydraulic pump to the steering cylinder. The steering system switches the steering valve based on the rotational direction and rotational quantity of the steering wheel so that the steering cylinder is controlled. In this case, the switching of the steering valve based on the rotational direction and rotational quantity of the steering wheel is performed by use of a hydraulic steering unit called “Orbit roll” (trade name). Here, the hydraulic steering unit includes: a hydraulic valve that operates in synchronization with the rotational operation of the steering wheel; and a hydraulic motor. The hydraulic steering unit is configured to generate a hydraulic pressure whose flow rate corresponds to the rotational quantity and rotational direction of the steering wheel.
In such a steering system for an engineering vehicle, typically, the hydraulic steering unit is located in a main circuit, and the steering valve is operated with the steering wheel to control the steering cylinder. In this case, a hydraulic valve of the hydraulic steering unit is provided as a steering valve, and the hydraulic pressure generated by the hydraulic valve unit is directly introduced into the steering cylinder.
In contrast to such a typical steering system for an engineering vehicle, a technology is known in which the hydraulic steering unit is used as an operation system of the steering valve so that the rotational operation of the steering wheel can be performed with a small force (for example, patent document 1, JP-U-1-154974).
On the other hand, there is also known a technology in which the operation system of the steering valve is formed by an electric/hydraulic steering unit including a controller and a solenoid valve so that the steering valve is electrically/hydraulically switched (for example, patent document 2, JP-A-10-45014). According to this technology, a rotational operation angle (steering angle) of the steering wheel is detected by a potentiometer, and the detected value is input to a controller. The controller outputs to the solenoid valve a command signal (electric signal) corresponding to the detection value. The solenoid valve, in turn, outputs a control pressure corresponding to the command signal. The control pressure is introduced into a hydraulic switching unit (pressure receiving unit) of the steering valve so that the steering valve is switched.
Patent document 1: JP-U-1-154974
Patent document 2: JP-A-10-45014